The present invention relates to an active array antenna system for use in wireless communication and comprising a plurality of element antennas and radio frequency circuits, and more particularly to a variable phase shifter circuit for controlling the phase of a local signal which is supplied to a frequency converting circuit in the radio frequency circuit.
This application is based on Japanese Patent Application No. 10-131982, filed May 14, 1998, the content of which is comprised herein by reference.
In general, the active array antenna system comprises a plurality of element antennas and radio frequency circuits connected to the element antennas. The active array antenna system is an antenna system for imparting an appropriate phase difference or the phase difference and an appropriate gain difference to a received RF signal or an RF signal to be transmitted, of each element antenna. Thus, directional beam scan can be performed or an arbitrary directional beam can be realized.
A conventional beam scan method adapted to the active array antenna system has been disclosed in Japanese Patent Laid-Open No. 7-202548 (hereinafter techniques described in this disclosure are called "conventional techniques"). According to the disclosure, a variable phase shifter circuit is provided for imparting a predetermined phase difference to a local signal in a carrier-wave frequency band which is supplied to each of frequency converting circuits corresponding to the plural element antennas. Since the S/N ratio of the local signal in the carrier-wave frequency band is higher than that of the received RF signal, the conventional technique attains the following advantages:
(1) An influence of deterioration in the S/N ratio caused by the variable phase shifter circuit on the RF signal can be limited as compared with a case where the variable phase shifter circuit is provided for a signal line for the RF signal.
(2) A plurality of variable phase shifters can concentrically be disposed.
(3) The structure of the control system can be simplified.
When the foregoing conventional technique is applied to a wireless communication system which uses a high carrier-wave frequency, such as a microwave or a millimeter wave, the foregoing conventional technique, however, encounters the following problem. That is, the cost of the variable phase shifter circuit for the local signal in the carrier-wave frequency band cannot be reduced. As a result, the overall cost of the active array antenna system cannot be reduced.
According to the conventional technique, the carrier-wave frequency is fixed. When the conventional technique is used to receive or transmit a plurality of carrier-wave frequencies by a single active array antenna system, such as the FDMA system or a multi-carrier TDMA system, there is a disadvantage that the structure of a power supply system becomes complex.
The conventional technique employs a filter or a delay element (for example, a delay line) to serve as the variable phase shifter circuit for the local signal in the carrier-wave frequency band. If the phase shift variation function is provided for the filter or the delay element, the cost cannot be reduced or a variable range for the phase shift is limited in general. As a result, beam scan freedom is narrowed.
As described above, the conventional active array antenna system has the structure that the phase of the local signal in the carrier-wave frequency band for the beam scan is controlled by the variable phase shifter circuit. When the conventional active array antenna system is applied to a wireless communication system using a high carrier-wave frequency, the cost of the variable phase shifter circuit cannot however be reduced. Thus, there arises a problem in that the cost of the active array antenna system cannot be reduced. Since the carrier-wave frequency is fixed, a plurality of carrier-wave frequencies cannot easily be transmitted or received by a single active array antenna system. Since the filter or the delay element is employed as the variable phase shifter circuit, the variable range of the phase shift is limited. As a result, there arises a problem in that beam scan freedom is narrowed.